Application of the spin trapping technique in intact animals requires an understanding of the stability of the spin traps and their spin adducts in vivo. We studied the stability of DMPO in vivo in mice using HPLC and the stabilityof spin adducts of DMPO by EPR in plasma, whole blood, peritoneal fluid, and ho mogenized heart tissue of the rat. DMPO was evenly distributed in the liver, heart, and blood of the mice 15 minutes after intraperitoneal injection and 40% remained in the organs 2 hours after the injection. In contrast, the spin adduct DMPO-OH was short-lived, with a half-life of 3.0 minutes in plasma, and was not detectable 1 minute after formation in whole blood and homogenized heart tissue.It also was found that the carbon-centered spin adduct DMPO-CH(OH)CH3 was more stable, having a half-life of 16, 11, 3.6, and 0.79 minutes in plasma, peritoneal fluid, whole blood, and homogenized heart tissue, respectively. These results indicate that the stability of the spin adducts is the limiting factor forin vivo applications of DMPO.